In a web conveyor device that feeds out, from a sending portion a physical object wherein the paper, film, cellophane, metal foil, rubber, or the like, is rolled into the shape of a roll (hereinafter termed a “web”), to perform a specific process on the web, and then to take up the web, in a receiving portion, after processing, it is necessary to control the velocity of movement of the web so as to be uniform, and to measure accurately the length of web that is fed out and taken up.
Conventionally there has been the method disclosed in Japanese Patent 4180369 (“JP '369”) as a method for measuring the velocity of a web. FIG. 21 is a block diagram illustrating the structure of the conventional velocity measuring device disclosed in JP '369. In this velocity measuring device, a laser beam that is emitted from a laser diode unit passes through a lens 201 to be focused, as a measuring beam 202, on a web 205, such as, for example, a paper sheet. If there is the web 205 in the path of the measuring beam 202, then the measuring beam 202 will reflect from the web 205 and will thus be scattered. One part of the emission of the measuring beam 202 that is scattered will trace the original path to be focused by the lens 201 onto the emission surface of the laser diode unit 200, to enter again into the laser resonator. The result is a change in the intensity of the laser emission. This change in intensity of the laser emission is detected by a photodiode within the laser diode unit 200, which performs conversion into an electric signal, and by an electronic circuit 203 that processes the electric signal.
The electronic circuit 203 forms a portion of a controller 204. The controller 204 controls the rotation of a roll 206, and thus controls also the movement of the web 205. When the web 205 moves, the return beam that is reflected from the web 205 experiences a Doppler shift. This means that the frequency of the return beam changes, or that a frequency shift occurs. This frequency shift is controlled by the velocity of movement of the web 205. The return beam that reenters the laser optical resonator interferes with the laser beam that is produced by the optical resonator. This interference means that a self-coupled effect occurs within the optical resonator. The intensity of the laser emission increases or decreases due to this interference.
Here, when an electric current that is driven in a repeating triangle wave that increases or decreases with a constant rate of change in respect to time is provided to the laser of the laser diode unit 200 as an driving current, the laser is driven so as to repetitively alternate between a first oscillating interval wherein the oscillating wavelength increases continuously at a constant rate of change and a second oscillating interval wherein the oscillating wavelength decreases continuously at a constant rate of change. The difference between the number of pulses that are included in the electric signal that is outputted from the photodiode during the first oscillating interval and the number of pulses included in the electric signal that is outputted from the photodiode in the second oscillating interval is proportional to the velocity of the web 205. Consequently, the electronic circuit 203 is able to calculate the velocity of the web 205 from the difference in the number of pulses.
However, in JP '369, the optical path length between the laser and the web 205 is relatively short, and the laser oscillating wavelength modulation frequency and amplitude are relatively small, so under a specific set of circumstances, such as the movement of the web 205 to be measured being relatively fast, the velocity of the web 205 will cease to be proportional to the difference in the aforementioned number of pulses. In this case, an average value is calculated for the number of pulses included in the electric signal that is outputted from the photodiode during the first oscillating intervals and the number of pulses included in the electric signal that is outputted during the second oscillating intervals, and a uniform constant is extracted from the average values, to confirm the velocity of the web 205.
As described above, in the conventional velocity measuring device disclosed in JP '369, it is assumed that the velocity of the web and the distance between the laser and the web are known roughly, and different methods for calculating the velocity are used depending on the state of the web. However, in the conventional velocity measuring device, disclosed in JP '369, if the velocity of the web and the distance to the web are not known, then it is not possible to use different methods for calculating the velocity depending on the state of the web, and thus there is a problem point in that it is not possible to calculate the velocity of the web correctly.
The present invention is to solve the problem set forth above, and the object thereof is to provide a velocity measuring device and method able to measure accurately the velocity of a web, through being able to handle the case wherein the velocity of the web and the distance between the laser and the web are unknown in a self-coupled velocity measuring device.